Now the world-line passing through P will be described by a substantial point with the constant _mechanical mass m_. Let us call _m-times_ the velocity-vector at P as the _impulse-vector_, and _m-times_ the acceleration-vector at P as the _force-vector of motion_, at P. According to these definitions, the following law tells us how the motion of a point-mass takes place under any moving force-vector[32]: _The force-vector of motion is equal to the moving force-vector._ This enunciation comprises four equations for the components in the four directions, of which the fourth can be deduced from the first three, because both of the above-mentioned vectors are perpendicular to the velocity-vector. From the definition of T, we see that the fourth simply expresses the “Energy-law.” Accordingly _c²_-_times the component of the impulse-vector in the direction of the t-axis is_ to be defined as _the kinetic-energy_ of the point-mass. The expression for this is _mc²_ _dt_/_d_τ = _mc²_ /√(1 - _v²_/_c²_) _i.e._, if we deduct from this the additive constant _mc²_, we obtain the expression ½ _mv²_ of Newtonian-mechanics up to magnitudes of _the order of_ 1/_c²_. Hence it appears that _the energy_ depends _upon the system of reference_. But since the _t_-axis can be laid in the direction of any time-like axis, therefore the energy-law comprises, for any possible system of reference, the whole system of equations of motion. This fact retains its significance even in the limiting case c = ∞, for the axiomatic construction of Newtonian mechanics, as has already been pointed out by T. R. Schütz.[33] From the very beginning, we can establish the ratio between the units of time and space in such a manner, that the velocity of light becomes unity. If we now write √-1 _t_ = _l_, in the place of _l_, then the differential expression _d_τ² = -(_dx²_ + _dy²_ + _dz²_ + _dl²_), becomes symmetrical in (_x_, _y_, _r_, _l_); this symmetry then enters into each law, which does not contradict the _world-postulate_. We can clothe the “essential nature of this postulate in the mystical, but mathematically significant formula 3·10⁵ _km_ = √-1 Sec. V The advantages arising from the formulation of the world-postulate are illustrated by nothing so strikingly as by the expressions which tell us about the reactions exerted by a point-charge moving in any manner according to the Maxwell-Lorentz theory.